A culture medium is a nutrient solution or solid preparation that provides the environment and nutrients required for microorganisms or cells to grow.
Explanation
Culture media supply essential nutrients, energy sources, minerals and water to support the growth of bacteria, fungi, algae or mammalian cells. Media may be liquid broths or solidified with gelling agents such as agar. Early media used gelatin, but gelatin melted at incubation temperatures and was degraded by many bacteria; agar, suggested by Fanny Hesse, became the standard because it remains solid between 32–42 °C, melts at about 85 °C, has good diffusion properties and is not digested by most organisms.
Basic ingredients include a carbon source (e.g., glucose), nitrogen sources (peptones, amino acids or ammonium salts), salts, trace elements and, for fastidious organisms, vitamins and growth factors. Media are classified as complex or defined. Complex media (such as nutrient broth or tryptic soy agar) contain extracts of yeast, meat or soybeans with undefined composition, while defined media contain known quantities of pure chemicals and support physiological studies. Selective media incorporate inhibitors to suppress unwanted microbes, whereas differential media include indicators to distinguish organisms based on metabolic traits. Enriched media contain additional nutrients to grow fastidious pathogens. In cell culture, media are formulated with serum, buffering agents and hormones to maintain mammalian cells. The pH of most microbial media is adjusted near neutrality; acidophiles and alkaliphiles require lower or higher pH values respectively. Sterilization by autoclaving or filtration is essential before use.
Types and Uses
- Broth media provide a homogeneous environment for rapid growth and biomass production.
- Agar-based media enable isolation of pure colonies and allow observation of colony morphology and pigment production.
- Selective media such as MacConkey agar or Sabouraud dextrose agar inhibit unwanted organisms while promoting target species.
- Differential media, for example blood agar, reveal haemolysis or other metabolic traits through indicators.
- Defined minimal media support growth of model organisms like E. coli for metabolic and genetic studies.
Choosing the appropriate culture medium is critical for isolating, identifying and studying microorganisms or for maintaining cell lines. Media composition affects growth rate, physiology and expression of traits, so careful formulation and sterile preparation are essential.
Related Terms: Nutrient Agar, Selective Medium, Defined Medium, Broth, Autoclave